Active and passive plasmonic waveguides for superior photonics applications
2017-02-28T00:33:30Z (GMT) by
Guiding optical energy in metal–dielectric nanostructures by the use of plasmon excitations known as surface plasmons has received much attention over the past few decades. The diverse applications of this technology span many areas in modern science, including scanning near-field optical microscopy (SNOM), bio-medical imaging and sensing, surface-enhanced Raman spectroscopy (SERS), and the realization of nanophotonic circuit elements. Plasmonic waveguides play a prominent role in the efficient operation of these devices, which are responsible for carrying optical signals in subwavelength dimensions. The guided optical modes suffer from propagation losses that arise due to various factors, such as scattering from surface imperfections in waveguides, absorption losses in dielectrics, and ohmic heating in metals. Even though scattering losses may be minimized by employing cutting-edge fabrication techniques that stem from the rapid advancements in material engineering, and dielectric losses are often negligibly small, the metal losses are high in magnitude and thus cannot be overlooked. Since metals are essential to sustain and guide the plasmonic modes, metal losses cannot be entirely eliminated. However, these losses may be compensated by doping the dielectric with rare-earth ions and providing optical gain via pumping. Since the amount of optical gain that can be supplied is practically limited, it is vital that waveguides are designed in such a way that the detrimental effects of metal losses are minimal. Waveguides of different geometrical shapes and arrangements have been identified as candidates for plasmonic waveguides, such as planar waveguides, circular cylinders, waveguides with square and triangular cross-sections, metal wedges and grooves, and linear chains of metal and metal–dielectric composite particles. These geometries have their own merits and demerits, in terms of the propagation losses and mode confinement. In this dissertation, the focus is on planar and circularly cylindrical geometries, and a number of both active and passive multi-layer structures are examined numerically, as well as analytically, for the efficient propagation of plasmonic modes. The effect of the geometrical parameters of the waveguide on propagation characteristics is investigated below the plasmon resonance frequency. Considering a planar waveguide consisting of a finite dielectric layer on a thick metal, it is shown that the guided mode experiences maximal modal gain at a particular thickness of the optically pumped dielectric layer. The threshold gain required to fully compensate for the losses (critical gain) in a metal–dielectric– metal (MDM) waveguide of infinite extent is estimated analytically, and an exact analytical expression for the confinement factor is derived. The more realistic case of an MDM structure with finite metal layers is also investigated, and it is revealed that thicknesses of metal/dielectric layers can be adjusted to ensure the furthest propagation of the guided mode. When the dielectric region is pumped to provide optical gain, the losses may be suppressed by minimal pump power at a particular choice of geometrical parameters. Additionally, it is shown that the gain experienced by the mode also becomes minimal, depending on the waveguide geometry. An exact analytical expression for the confinement factor is also presented. For a dielectric–metal–dielectric waveguide capped with metal, an approximate analytical solution for the dispersion equation is derived. The optimal geometrical parameters that yield the furthest propagation of the mode and compensation of losses with minimal optical gain are estimated analytically. Furthermore, approximate analytical expressions for the critical gain and the confinement factor are derived. Several composite cylindrical nanowire structures are also investigated for plasmonic guiding. For a nanowire consisting of a dielectric core and a metal cladding, it is shown that the critical gain becomes minimal at a particular cladding thickness. Similarly, the geometrical parameters of metal-core dielectricclad nanowires can also be chosen to lower the material gain requirement. Cylindrical MDM nanowires are also investigated, and it is shown that the guided mode can be strongly confined within the dielectric layer. The existence of optimal nanowire geometry that enables maximum propagation length of the mode and compensation of metal losses with minimal material gain is found.